1. Field of the Invention
The present invention relates to an automotive torque distribution control system, and specifically to a system which is applicable to actively vary a distribution ratio of a portion of the torque delivered to a pair of subsidiary drive wheels (e.g., front road wheels) to the remaining portion of the torque delivered to a pair of primary drive wheels (e.g., rear road wheels), to actively vary a torque-distribution ratio between left and right road wheels by varying a differential limiting force of a differential mechanism interposed between the left and right road wheels, or to actively vary a torque-distribution ratio between front and rear road wheels by varying a differential limiting force of a center differential mechanism.
2. Description of the Prior Art
In recent years, there have been proposed and developed various automotive traction control systems which are adopted on four-wheel-drive vehicles with a torque-split transfer device, or a four-wheel-drive vehicles with a center differential interposed between front and rear road wheels or on a vehicle with a limited slip differential interposed between right and left road wheels, in order to enhance the driving stability of the vehicle and to secure a good traction. Such automotive torque distribution control systems have been disclosed in Japanese Patent Provisional Publication Nos. 1-229722 and 3-31031 both assigned to the assignee of the present invention. For instance, on four-wheel-drive vehicles with a typical torque distribution control system in combination with a transfer clutch, the system serves to varying the engaging force of the transfer clutch in response to a wheel-speed difference between front and rear wheel speeds, so as to prevent wheel-spin when accelerating or starting rapidly, to prevent wheel-lock which might occur at either one of left and right road wheels during hard braking, and/or to prevent wheel-slip during driving on so-called low-.mu. roads or split- .mu. roads. As is generally known, the greater the wheel-speed difference, the greater the engaging force of the transfer clutch, thus permitting the vehicle to shift from two-wheel-drive to four-wheel-drive. This can prevent excessive torque from being delivered to the road wheels and provide adequate traction depending on road surface conditions. Suppose that the outside diameter of either one of four tires is remarkably different from the other, for example owing to excessively worn tire, a different type or kind of tire located on the vehicle, fluctuations in manufacturing quality of tires, or an emergency tire often called "temper tire" installed on the car in case of emergency. In such a case, the conventional system decides that there is a difference between the rear- and front-wheel speeds, irrespective of during normal straight-ahead driving at a constant speed. The system tends to operate as if the vehicle experiences wheel-slip, even when there is less wheel-slip during normal straight-ahead driving. Generally, the system would rapidly increase the engaging force of the friction clutch of the transfer owing to the previously-noted wheel-speed difference based on installation of the tire of an outside diameter different from the other. As a result, excessive torque may be delivered to one of the front- and rear-ends of the vehicle. The undesired erroneous delivery of excessive torque might induce a new wheel-slip, and thus the system operates to release the transfer clutch in such a manner as to change from four-wheel-drive tendencies (a greater engaging force of the clutch) to two-wheel-drive tendencies (a less engaging force of the clutch). This results in hunting of the torque distribution control in the system. As appreciated, in the event that there is a comparatively great difference of outside diameters of the tires, the transfer clutch is continually maintained at its engaged position with a comparatively great clutch engagement force. This would lower the driving stability of the vehicle and degrade fuel consumption. This would also result in undesired oil-temperature rise in the torque-transmitting line or power-train components. Such temperature rise could work disadvantageously to the power-train components from the viewpoint of heat resistance. To avoid this, the previously-described Japanese Patent Provisional Publication No.1-229722 discloses a torque distribution control system which is capable of regulating the rate of change in an engagement force of a transfer clutch with respect to t (time), while making an estimate of hunting of the clutch engagement force, in response to the state of variations in a command torque or a command clutch-engagement force computed and determined by a controller incorporated in the system. The system of the Japanese Patent Provisional Publication No. 1-229722 merely reduces the rate of change in the clutch engagement force with respect to t (time) when the system estimates that hunting may start to develop. On the other hand, the Japanese Patent Provisional Publication No. 3-31031 teaches the compensation of a wheel-speed difference between the rear- and front-wheel speeds, considering the wheel-speed difference which might take place, owing to a tire of an outside diameter different from the other, during constant-speed straight-ahead driving. Actually, in the system disclosed in the Japanese Patent Provisional Publication No. 3-31031, the wheel-speed difference is detected on the basis of signals from a plurality of wheel-speed sensing devices, indicative of rear- and front-wheel speeds. The wheel-speed difference detected under a specified traveling condition is set at a so-called dead-band difference. A corrected wheel-speed difference is then computed by subtracting the dead-band difference from wheel-speed difference data, and thus the engagement force of the transfer clutch is controlled in response to a command signal value based on the corrected wheel-speed. However, there is a possibility that the system disclosed in the Japanese Patent Provisional Publication No. 3-31031 erroneously determines that the wheel-speed difference takes place due to a tire of an outside diameter different from the other even when the wheel-speed sensing device located on the front wheel side is in trouble and there is no current signal from the front wheel-speed sensing device. In this case, the system would malfunction so that driving torque might be delivered to the front wheels.